Flywheels are increasingly being used as compact power storage devices, particularly in vehicles. The flywheel is mounted in a housing, the interior of which is maintained at low pressure to minimise drag on the flywheel as it rotates. Whilst this low pressure environment can readily be achieved before sealing the housing, fibre composite materials are often used in the construction of high energy density flywheels which emit gases long after fabrication, making it difficult to maintain the desired near vacuum conditions.
U.S. Pat. No. 5,462,402 describes a flywheel energy storage system including a flywheel disposed within a first chamber. A second chamber is provided which maintains a relatively high pressure. A shaft is suspended between a first bearing located in the first chamber and a second bearing in the second chamber. A molecular pump is driven by the shaft to pump gas molecules from the first chamber to the second chamber to maintain a low pressure environment in the first chamber.